Statuses with the same name don't stack. Tidal Roar won't stack with weakens attack because they both inflict "attack down". Both will stack with bio (which lowers attack) because it inflicts "bio" not "attack down".
Dia, Defense Down, and Box Step will all stack because they're three different effects.
Specific mobs might be immune, but the defense down on the polearm seems to proc at the same rate on tough NMs as it does on trash mobs, so it doesn't appear to take into account magic evasion or level difference or anything. I assume all the other downs follow the same mechanic.